What do you mean by "the best I can expect"? Resolution? Colours? Dynamic range? Composition? Light? Subject choice. Assuming you mean firstly resolution:

- this shot was at f8. But at f8, due to diffraction, you're losing out on effective resolution, and you can only properly feed about 29mp's for yellow-green light, and only 19mp's for red light. Only blue has a short enough wavelength that it does not get affected at f8 for this sensor size. If you want to have a chance to properly feed 36mp's then you should not go for an aperture smaller than f5.6. And that's with an ideal lens

- if you want to test your camera for ultimate resolution then obviously you should have the very best shooting conditions. The Tamron 28-70 f2.8 is a very good lens within this category - but of course even average primes beat it hands down on dimensions such as resolution, not to mention light transmission where a good prime loses you maybe 1.4EV whereas a good 24-70 zoom loses you perhaps 3 to 4EV.

- also if you want that kind of testing then you need camera on tripod, timer release, and ideal subject i.e. likely immobile aka landscape

- if it's the best colours you're after then this shot with the grey-ish sky probably isn't a good test shot

- if it's dynamic range you're after then again this shot under fairly dull light and with modest contrasts, again probably isn't a good test shot

Finally there are the artsy, intangible things of photography. For me this shot doesn't "do" it, it doesn't tell a story, it doesn't look like the work of an experienced photographer handling a great camera equipped with a solid lens but rather it looks like a snapshot which would have been more easily done with a smartphone. But again, this is art so tastes do vary and the fact that this shot doesn't work for me, doesn't mean that it might not work for someone else.

To me it feels like you're still at the stage where your big D800 coupled to that big lens, is limiting your creative freedom. Personally with my DSLR I found that at the very least several thousand shots were needed before it started to become like an extension of my eye, as opposed to a cumbersome tool - and even so and today after easily over 75'000 shots with it, I often leave it at home because while it might deliver better quality, it is too heavy / bulky / conspicuous for many places I go to - and we're talking only about a modest D7000, not a D800 coupled to a big lens.